C#多线程同时执行时耗时比单个线程执行耗时会增加

运行结果： 

 源码：

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Control.CheckForIllegalCrossThreadCalls = false;
        }

        private object obj;
        private void ceshiThread(Object obj)
        {
                Stopwatch sp = new Stopwatch();
                sp.Restart();
                int index = (int)obj;
                int count = 0;
                for (int i = 0; i < 10000; i++)
                {
                    for (int j = 0; j < 10000; j++)
                    {
                        int value = i * j;
                        count++;
                    }
                }
                sp.Stop();
                textBox1.AppendText(string.Format("线程 {0} 执行次数 {1} ，总耗时 {2}", index, count, sp.ElapsedMilliseconds.ToString()));
                textBox1.AppendText(Environment.NewLine);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //int a = 0;
            //while (a < 100)
            {
                for (int i = 0; i < 4; i++)
                {
                    Thread th = new Thread(ceshiThread);
                    th.IsBackground = true;
                    th.Start(i);
                    //Task t = new Task(() => ceshiThread(i));
                    //t.Start();
                }
                //a++;
                //Thread.Sleep(5);
            }

        }

        private void button2_Click(object sender, EventArgs e)
        {
            //Parallel.For(0, 4, (i) =>
            //{
            //    ceshiThread(i);
            //});

            for (int i = 0; i < 4; i++)
            {
                ThreadPool.QueueUserWorkItem(ceshiThread, i);
            }
            //Task t = new Task(() => ceshiThread(0));
            //t.Start();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            //for (int i = 0; i < 100; i++)
            //{
                Thread th = new Thread(ceshiThread);
                th.IsBackground = true;
                th.Start(1);
                //Thread.Sleep(200);
            //}
            //Thread th = new Thread(ceshiThread);
            //th.IsBackground = true;
            //th.Start(1);
        }

    }
}